如何使用下一个可观察的对象并在我的解析器中完成for循环

Question

接下来如何使用可观察的并在角度7中完成for循环

for (const invoice of activityInstance.dataEntries['invoices'].value) {
     var eciObjectIdentifier = {
     'id': invoice.identifier.id,
     'sourceName': 'dss-default-source'
     };

this.eciContentService.getDocumentContentParts(this.sessionService.getPrincipal(), eciObjectIdentifier).subscribe((contentParts) => {
    invoice.contentParts = contentParts;
    eciObjectObservable.next(this.activityInstance);
    eciObjectObservable.complete();
      });
    }